In the field of filtration of hazardous materials from large scale airflow systems, and more specifically in the field of removal, disposal and replacement of contaminated filter elements, typical known techniques involve removal of the filter element itself from within its essentially permanently fixed housing, and replacement by a new filter element in the same housing. Perhaps the most prevalent known techniques have emphasized the use of plastic bags to contain the filter element during removal and handling as the filter element is removed from the fixed housing. Perhaps the leading known system is what is referred to as the "bag-in, bag-out" technique of filter replacement. This technique is disclosed in U.S. Pat. No. 3,354,616 issued Nov. 28, 1967, and similar processes are disclosed in U.S. Pat. Nos. 4,334,896 issued Jun. 15, 1982, and 3,402,530 issued Sep. 24, 1968. Generally, the "bag-in, bag-out" technique involves the use of a filter housing unit integrated into the exhaust duct system, the housing containing one or more filter elements and having an opening communicating with the location of the filter element. About the opening is affixed the end of a plastic bag, thereby sealing off the enclosure to the outside. A more permanent lid is also generally provided for sealing over the plastic bag whereby the bag is compacted for storage behind the lid while still affixed to the mouth of the opening. With the rigid lid removed, the operator reaches in for the filter element, and is protected from the contaminated air by the plastic bag. He then manipulates the filter element through the opening and into the bag. The entire filter element is thus enclosed by the plastic bag, and the bag is then crimped between the filter element and the mouth of the housing opening, and heat sealed along that line. This heat sealing creates a complete enclosure of the filter element within the bag, and the bag may then be cut along the line of the heat seal, separating the bagged filter element from the housing, with the housing remaining sealed by virtue of the heat sealed stump of the plastic bag affixed over the mouth of the housing opening. Another bag containing a new filter element is then sealed over the existing plastic stump on the mouth of the opening, and the stump is removed and heat sealed off into a small pocket of the new plastic bag, after which this pocket is cut off and disposed of. The new filter is then moved through the opening out of the plastic bag and into position within the housing. The new plastic bag is then rolled up and placed for storage behind the solid lid which has been secured over the opening. The "bag-in, bag-out" technique described above, while rather widely used, does have certain disadvantages. In particular, it may be extremely awkward and difficult to handle large filters by hand, especially carbon filters which may exceed 200 pounds in weight. Additionally, the use of large plastic bags and the need to manipulate the filters through such bags presents the risk that the bags (upon which the safety of the procedure relies) may be easily punctured by the large filter elements or by simple operator mishandling. The reliance upon the heat sealing of the plastic bags presents a further potential for imperfect sealing and failure-of the plastic enclosure.
The present invention addresses the problems of the "bag-in, bag-out" technique by replacing the large permanent fixed filter housing with a removable and disposable filter module comprising both a filter element or elements and a disposable housing for the element or elements. The invention thus involves the provision of filter modules for use in this particular field which can be safely, quickly and efficiently, as well as economically, removed and disposed of as integral units, and simultaneously replaced by similar filter modules. A particular feature of the invention involves the method by which a used disposable filter module can be removed and replaced safely and without exposure of the technician to hazardous materials in the module itself and in its associated ducting. Thus, the invention involves both method and apparatus aspects. I am aware that small disposable filter canisters for use in or with HEPA-filtered vacuum cleaners are known in the published prior art (e.g., U.S. Pat. Nos. 4,613,348 issued Sep. 23, 1986, and 4,726,825 issued Feb. 23, 1988), for use in conjunction with a separate vacuum cleaner device having a suction hose, or for use with a portable vacuum motor assembly removably mounted on top of the canister, typically for use in entrapping asbestos-containing dust. The only such examples of which I am aware relate to portable vacuum cleaners or the like, of a relatively small size, and I am not aware of any proposal for the modification of such disposable canisters to the size scale and for permanent ducting installations of the type to which the present invention relates. I would consider it unlikely that persons skilled in the art to which the present invention relates would view known small disposable filter canisters for vacuum cleaners as providing any concept or solution in connection with the problem of coming up with an improved alternative to the established "bag-in, bag-out" techniques described above in systems to which those techniques relate.